Vesicular membranes are intimately involved in the storage, transport and release of regulatory and metabolic agents in cellular systems. To function properly these vesicles must be able to actively accumulate these agents, retain them for periods of time and then expel them to an external environment by fusion with a limiting membrane. It is our objective to understand at a molecular level factors which affect these stability, fusion and solute concentration properties and to use that understanding in the design of useful artificial analogues. These objectives will be pursued largely through the study of model phospholipid vesicles. Vesicle fusion as stimulated by divalent cations in mixed anionic lipid systems will be studied using a variety of physical techniques. Emphasis will be placed on the study of structural and dynamic properties on the phosphatidic acid-phosphatidylcholine-Ca ions system using 31P, 113Cd and 43Ca nmr. Permeation of small solutes through vesicle membranes will be studied using newly developed spin population transfer and rapid sequential accumulation techniques. Application of these same techniques to natural vesicle systems will be explored.